1. Field of the Invention
The present invention is related to a brushless DC motor driving system, and more particularly, to a brushless DC motor driving system having good linearity, which is used to achieve good low rotation-speed output performance and linear motor rotation speed control, thus enabling the application of such brushless DC motor driving system in Blue-Ray DVD ROM or Light Scribe DVD Player.
2. Description of the Prior Art
Regarding the previous technology for the three-phase motor driving method, related patents such as patents No. CN1989689A and No. CN1914789A have described the motor control method and peek current protection circuit, wherein the synthesis circuit generates different phase modulation signals by phase modulation of the input PWM (Pulse Width Modulation) control signal, and then the PWM output comparator amplifies the phase modulated signal and outputs the amplified signal to the motor control circuit to drive the outside power driving transistor; finally the function of motor control is achieved by driving the motor coil circuit. In addition, for the stability of the rotation speed of motor to be achieved, when the current on the motor coil increases, the rotation speed of motor also increases. Then the Hall elements would sense the current on the coil, transform the sensing current to the sensing voltage, feed back the sensing voltage to the Hall differential control circuit, and adjust the PWM control signals. Meantime, the detection voltage would compare with the reference voltage and input rotation speed control voltage through the feedback comparator, and stable rotation speed control can be achieved.
As shown in FIG. 1, the motor control circuit using a known PWM control method, which comprises a driving motor 2, a power driving transistor 7 and PWM driving control circuit 6, in which the PWM drive control circuit 6 consists of the hall differential amplifier 16, PWM synthesis circuit 17, PWM comparator amplifier 18, reference voltage comparator 24, input rotation speed comparator 13, current detection resistance 12, oscillation prevention capacitor 22, and motor driving control circuit 20. The characteristic of the motor control circuit is to use the PWM signal of the input motor driving control circuit to minus the output RL of the reference voltage comparator and obtain the more real PWM drive control signal for the reason of eliminating the additional conduction time generated by the delay from the oscillation prevention capacitor. The output RL of the reference voltage comparator is generated from the difference between the output of the peek retention circuit and reference voltage, and the input voltage of the peek retention circuit is determined by the current detection resistance 12 and the conduction current controlled by the PWM control signals. During the turn on time of the PWM control signals, the peek retention circuit determines the detection value of the peek voltage, and is discharged with a certain time constant during the off time of the PWM control signals.
Moreover, the input terminal of the input rotation speed comparator 13 is connected with peek detection voltage, reference level voltage, and input control voltage (SIG), so that the lower square wave of the reference level voltage and input control voltage (SIG) is compared with the peek detection voltage. When the input control voltage (SIG) is higher than the peek voltage of the peek retention circuit 14, the output voltage will increase as output voltage (U,V,W) of the synthesis circuit also increases, and then the turn on time of the output PWM control signals of the PWM output comparator 18 in duty cycle also increases. The PWM control signals then pass through the motor driving control circuit 20 to the power driving transistor 7 and drive the three-phase coils on the driving motor, and therefore the driving current on the coil increases and the rotation speed of the motor also increases. Then the driving current passes through the current detection resistance 12 and generates a voltage to be compared with the rotation speed control voltage. After a sequence of feedback control steps, the peek voltage and rotation speed control voltage will become stable.
And vice versa, when the input control voltage decreases, the rotation number of the driving motor 2 also decreases, and the decreased peek voltage will become in accordance with the rotation speed control voltage after the above sequence of control steps is repeated.
However, when the driving motor 2 is overloaded, the rotation number detected by the rotation speed counter decreases. In order to increase the rotation speed of driving motor, the input control signals will be intensified, and the rotation speed of the drive motor 2 also increases, which results in abnormal increase of the rotation control voltage. When the rotation speed control voltage exceeds the reference control voltage, the reference voltage will replace the rotation speed control voltage and become the object of comparison of the input peek voltage of the rotation speed comparator 13, the highest rotation speed thus being limited to avoid excessive driving current which leads to burning out of the current coil of the driving motor. In addition, when the rotation speed voltage exceeds the determined value, once the output voltage of the peek retention circuit becomes higher than the reference voltage, the output of the reference voltage comparator will be at high level, and the additional output turn on time of the PWM output signals of the motor driving control circuit is eliminated. The stability is further strengthened for the rotation speed control voltage and rotation speed of the drive motor.
As in the above-mentioned prior art, the duty cycle of the PWM control signals is determined by the difference between the input control signal (SIG) and the output voltage of the peek retention circuit, and the turn on time of the high electric level is determined by the difference between output voltage of the peek protection circuit and the reference voltage, and finally the consistency is achieved between the duty cycle of PWM control signals and the expected output rotation speed. However, the signal source and signal difference of the PWM control signals synthesized from the difference of the input control signal (SIG) and peek retention circuit and the Hall differential signal are inputted from different feedback paths. Although the input signal of the motor driving control circuit 20 can eliminate the excess delay time of the PWM control signals, the signal is easily induced by the non-linear distortion because of the noise source from different feedback paths.
Since the Blue-Ray DVD ROM is becoming the main stream in the future market, the system requirements on the precision of servo control of the Blue-Ray DVD ROM is higher than those of traditional CD and DVD in regard of the advancement of compression technology and increase of storage capacity. Overall, the control system of the Blue-Ray DVD ROM mainly includes the optical pickup system, servo control, and optical system design. As the servo control is related to the stability of the spindle motor driving system for driving the rotation of the disc and the control of motor driving rotation speed, the circuit design for the motor driving of Blue-Ray DVD ROM of higher storage capacity becomes more important. And more particularly, in the process in which the Blue-Ray DVD ROM picks up signal, higher and stable pick-up speed is needed to facilitate data processing in the optical pick-up process, and the motor must also keep at high driving speed and maintain certain linearity. Since the Blue-Ray DVD ROM needs lower and relatively more stable control in the process of light scribe, particular emphasis needs to be lay on the stability of linearity of the motor driving circuit.
In order to overcome the non-linear distortion problem of the feedback path described above, the present invention provides a brushless DC motor driving system which uses the difference between the input signal and the reference voltage to determine the duty cycle and the turn on time of the PWM control signals; at the same time, the feedback controls of the rotation speed are on the same path, and therefore the best linear ratio of the input voltage to the output rotation speed is obtained.